Space Travel via Ions

For 60 years, NASA has used chemical rockets to send its astronauts into space, and to get its spacecraft from planet to planet. The huge million-pound thrusts sustained for only a few minutes were enough to do the job, mainly to break the tyranny of Earth’s gravity and get tons of payload into space. This also means that Mars is over 200 days away from Earth and Pluto is nearly 10 years. The problem: rockets use propellant (called reaction mass) which can only be ejected at speeds of a few kilometers per second. To make interplanetary travel a lot zippier, and to reduce its harmful effects on passenger health, we have to use rocket engines that eject mass at far-higher speeds.

In the 1960s when I was but a wee lad, it was the hey-day of chemical rockets leading up to the massive Saturn V, but I also knew about other technologies being investigated like nuclear rockets and ion engines. Both were the stuff of science fiction, and in fact I read science fiction stories that were based upon these futuristic technologies. But I bided my time and dreamed that in a few decades after Apollo, we would be traveling to the asteroid belt and beyond on day trips with these exotic rocket engines pushing us along.

Well…I am not writing this blog from a passenger ship orbiting Saturn, but the modern-day reality 50 years later is still pretty exciting. Nuclear rockets have been tested and found workable but too risky and heavy to launch. Ion engines, however, have definitely come into their own!

Most geosynchronous satellites use small ‘stationkeeping’ ion thrusters to keep them in their proper orbit slots, and NASA has sent several spacecraft like Deep Space-1, Dawn powered by ion engines to rendezvous with asteroids. Japan’s Hayabusha spacecraft also used ion engines as did ESA’s Beppi-Colombo and the LISA Pathfinder. These engines eject charged, xenon atoms ( ions) to speeds as high as 200,000 mph (90 km/sec), but the thrust is so low it takes a long time for spacecraft to build up to kilometer/sec speeds. The Dawn spacecraft, for example, took 2000 days to get to 10 km/sec although it only used a few hundred pounds of xenon!

But on the drawing boards even more powerful engines are being developed. Although chemical rockets can produce millions of pounds of thrust for a few minutes at a time, ion engines produce thrusts measured in ounces for thousands of days at a time. In space, a little goes a long way. Let’s do the math!

The Deep Space I engines use 2,300 watts of electrical energy, and produced F= 92 milliNewtons of thrust, which is only 1/3 of an ounce! The spacecraft has a mass of m= 486 kg, so from Newton’s famous ‘F=ma’ we get an acceleration of a= 0.2 millimeters/sec/sec. It takes about 60 days to get to 1 kilometer/sec speeds. The Dawn mission, launched in 2007 has now visited asteroid Vesta (2011) and dwarf planet Ceres (2015) using a 10 kilowatt ion engine system with 937 pounds of xenon, and achieved a record-breaking speed change of 10 kilometers/sec, some 2.5 times greater than the Deep Space-1 spacecraft.

The thing that limits the thrust of the xenon-based ion engines is the electrical energy available. Currently, kilowatt engines are the rage because spacecraft can only use small solar panels to generate the electricity. But NASA is not standing still on this.

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The NEXIS ion engine was developed by NASA’s Jet Propulsion Laboratory, and this photograph was taken when the engine’s power consumption was 27 kW, with a thrust of 0.5 Newtons (about 2 ounces).

An extensive research study on the design of megawatt ion engines by the late David Fearn was presented at the Space Power Symposium of the 56th International Astronautical Congress in 2005. The conclusion was that these megawatt ion engines pose no particular design challenges and can achieve exhaust speeds that exceed 10 km/second. Among the largest ion engines that have actually been tested so far is a 5 megawatt engine developed in 1984 by the Culham Laboratory. With a beam energy of 80 kV, the exhaust speed is a whopping 4000 km/second, and the thrust was 2.4 Newtons (0.5 pounds).

All we have to do is come up with efficient means of generating megawatts of power to get at truly enormous exhaust speeds. Currently the only ideas are high-efficiency solar panels and small fission reactors. If you use a small nuclear reactor that delivers 1 gigawatt of electricity, you can get thrusts as high as 500 Newtons (100 pounds). What this means is that a 1 ton spacecraft would accelerate at 0.5 meters/sec/sec and reach Mars in about a week! Meanwhile, NASA plans to use some type of 200 kilowatt, ion engine design with solar panels to transport cargo and humans to Mars in the 2030s. Test runs will also be carried out with the Asteroid Redirect Mission ca 2021 also using a smaller 50 kilowatt solar-electric design.

So we are literally at the cusp of seeing a whole new technology for interplanetary travel being deployed. If you want to read more about the future of interplanetary travel, have a look at my book ‘Interplanetary Travel: An astronomer’s guide’, which covers destinations, exploration and propulsion technology, available at Amazon.com.

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7 thoughts on “Space Travel via Ions”

Great article. Ion engines certainly look like the way to go for deep space exploration. As you point out input power is the critical issue to be resolved. If, as you suggest, we could develop a “… small nuclear reactor that delivers 1 gigawatt of electricity …” we would be pretty much home free. But to put that kind of power generation capability into perspective it is instructive to note that the largest nuclear power plant in the United States, the Palo Verde Nuclear Station in Arizona, generates a maximum of only 3.8 GW of electricity from three 1.27 GW reactors! It’s hard to see how the efficiency of a fission reactor could be increased sufficiently to allow that kind of power generation capacity to be packaged in a reactor small enough to fit on a 1 ton spacecraft. Even a fusion reactor (assuming we could build one) would only increase efficiency by a factor of 4 or 5. I think it may be a while before we can fly to Mars in a week.

Dreaming is a good thing. Dreaming frequently leads to inspiration and inspiration provides the foundation for actions which propel us forward.

So here’s my thought on how we might proceed. It may very well prove impossible to pack a GW reactor into a box small enough to mount on a small (1 ton) space probe. If it turns out this is the case then we should look at the feasibility of increasing the efficiency (i.e., the thrust to input power ratio) of the ion engine itself.

Is this approach practical? The information presented in the blog is encouraging. The Culham Lab ion engines produced 2.4 N of thrust for 5 MW of input power or 0.00048 N/kW. By comparison, the Deep Space I engine produced 92 mN of thrust for 2.3 kw of input power. That works out to 0.04N/kW; almost 100 times more efficient than the Culham engine. So it appears there may be room for significant improvement in the performance of large ion engines through design optimization. Whether it’s by a factor of 10, 100, 1000, or more remains to be seen.

The real trick is to get enough power generated without using such a massive reactor that it defeats itself. In one of my SF novels, I suggested fusion, which certainly generates energy [hopefully, net energy 🙂 ] but the mass that accompanies it could be a problem. Basically, we still need a lot more research. We have ideas, but we have a long way to go before they are hugely successful.

Hi Ian, thanks for your thoughtful comments as always! Of course my article is predicated on the idea that fission/fusion systems will not be deployed because of political issues. A nuclear thermal system in which fuel is passed through a reactor core (open cycle) or heated by the reactor with no contact (closed cycle) are also excellent designs, but the political resistance to launching such systems is formidable. So ion technology seems the easiest to continue developing, however the issue of the power generating system is still at the core of the problem and solar panels simply don’t cut it for the larger 100-kilowatt systems. One way or the other, we need nuclear power in space. The ion engine design issues relate to erosion of the accelerating grid at the high particle throughputs needed for getting the highest thrusts (mass transport rates). I discussed some of this in my book ‘Interplanetary Travel:An astronomer’s guide’.

I just did a quick search on small reactor systems. The Babcock&Wilcox mPower system https://en.wikipedia.org/wiki/B%26W_mPower produces 160 megawatts in a 15×75 foot cylindrical system. The reactor core is only a few meters on a side. Launched in parts and assembled in space would work. You don’t need shielding from the crew if it is placed far enough from the ship.Some small reactor designs use the thorium cycle, and thorium can be mined on the moon where it is abundant in the regolith.The Hyperion Power Module weighs 50 tons complete, and produces 25 megawatts, which still gets us an ion engine design that could reach 10-15 Newtons (2-4 pounds) of thrust very roughly, which is a very significant improvement over current NASA kilowatt systems.

The rocket design concepts are too above my head to comment on but the idea to build in space and mine from the moon remind me of Gerard K. O’Neill’s vision in his book ‘The High Frontier’. I love the dream in spite of today’s realities.

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